As indicated in FIG. 10, this type of apparatus involves a reservoir 2, formed by a dam 1, being connected to stationary fishway 4, formed from a downstream side, via guiding waterway 3, with swing fishway apparatus 5 provided at an intermediate location in the waterway. Water pressure chamber 6 is provided in said waterway, and a fishway device body 7 is housed within said hydraulic chamber and supported by the downstream end (shaft 7a). The upstream end of the fishway device body 7 is lifted by winch 8. A plurality of dividing walls 9 are provided in an upper portion of the fishway device body 7. Fishway device body 7 is tilted corresponding to a water level of reservoir 2 with shaft 7a of the downstream end serving as a fulcrum. In this case, the operator must check the water level and then operate winch 8 to lift fishway body 7 and tilt it to match an upstream water level, so the apparatus is difficult to use, since it responds slowly to fluctuating water levels.
Japanese Utility Model Publication No. 59-17941 discloses an invention in which a fishway device body inclines to match an upstream water level automatically without the use of winch 8. As indicated in FIGS. 11, 12 and 13, frame 11, which provides small openings between side walls 10 on both sides of the waterway, is supported by a shaft on the downstream end while allowing to swing freely. One end of connecting tube 12, which is opened in the bottom of frame 11, opens into the upstream water. Floats 13 in the shape of plates are provided vertically at constant intervals on frame 11. Regulatory chambers 14 are provided within floats 13 and water is fed to the chambers 14 from water feed ports 15 to regulate the buoyancy of floats 13. Therefore, frame 11 is supported by the buoyancy of floats 13 and the water pressure which acts on a bottom surface of frame 11.
Since frame 11 is not floating in the water solely by floats 13, an angle of inclination of frame 11 is set by regulating the buoyancy by introducing water into regulatory chambers 14 from water feed ports 15 at installation.
However, since the buoyancy fluctuates according to the angle of inclination, when the water level in the reservoir rises, frame 11 rises too high resulting in a decrease in an amount of water which overflows floats 13. Thus, when the depth of the overflow water is insufficient, it is not suitable as a fishway. In addition, when the overflow water level is high, an angle of inclination of frame 11 becomes too small in that the water level in the reservoir lowers, resulting in an excessive amount of overflow water. Thus, a fishway is desired which achieves a suitable overflow water depth regardless of the height of the upstream water level.